The present invention is directed to a discharge lamp driving device with an abnormality detection and protection function of detecting a lamp life end for circuit protection.
(First Prior Art)
FIG. 1 s a circuit diagram showing one example of a prior discharge lamp driving device which is identical in circuit configuration to that shown in FIG. 36 of Japanese Patent Publication No. 8-251942. A rectifier DB composed of a diode bridge is connected to an AC power source AC through a surge absorption element ZNR and a filter circuit F. Connected across a pulsating output terminals are a high frequency bypassing capacitor C2, a series combination of switching elements Q1 and Q2 in the form of field effect transistors through a series circuit of diodes D5 and D6, a series combination of a smoothing capacitor C10 and a diode 13, and a high frequency bypassing capacitor C11. A series circuit of an inductor L2 and a diode D12 is connected between a connection point of switching elements Q1 and Q2 and a connection point of smoothing capacitor C10 and diode D13. A leakage transformer LT1 has a primary winding N1 that is connected in series with a DC blocking capacitor C3 between the cathode of diode D5 and the connection point of switching elements Q1 and Q2. A secondary winding N2 of the leakage transformer LT1 has its one end connected through a DC blocking capacitor C9 to one of filaments of one discharge lamp La1, and has its other end connected to one of filament of the other discharge lamp La2. The other filaments of the two discharge lamps La1 and La2 are connected at one ends thereof to each other through an auxiliary winding N3 of the leakage transformer LT1 and a DC blocking capacitor C6. The other ends of the filaments of the discharge lamps La1 and La2 are connected to each other through a resonant inducing capacitor C7. Further, a harmonic distortion improving capacitor C4 is connected across diode D6.
The two switching elements Q1 and Q2 are driven by a control circuit CNT to turn on and off alternately. The leakage transformer LT1 includes an auxiliary winding N4 for detection of lamp voltage of the discharge lamps La1 and La2. The detected voltage induced at the auxiliary winding N4 is rectified by means of a diode D8 and is fed to a detection circuit 20 for detection of the lamp voltage. Based upon thus detected lamp voltage, the control circuit CNT varies a switching frequency of the switching elements Q1 and Q2. In short, the source AC voltage is rectified through rectifier DB of which pulsating output is partially smoothed out by a valley-filling power source in the form of a step-down chopper circuit composed of switching element Q2, diode D12, inductor L2, smoothing capacitor C10 and a parasitic diode of switching element Q1. The partially smoothed DC output is converted into a high frequency output by means of an inverter circuit in the form of a half-bridge type including the switching elements Q1 and Q2. The high frequency output is fed through the leakage transformer LT1 to the discharge lamps La1 and La2 as a load for driving the same. Further, in this prior art, the harmonic distortion improving capacitor C4 compensates for a voltage difference between the rectifier DB and the valley-filling power source, while an input voltage is switched on and off by utilization of a high frequency voltage appearing within the inverter circuit so as to draw in the input current from the rectifier DB through a resonant circuit composed of leakage transformer LT1, capacitor C3, discharge lamps La1 and La2, and capacitor C7, and through capacitor C4 for improving harmonic distortion of the input current. The operation of this prior art is known and therefore not discussed herein.
When the above prior art sees that the discharge lamps La1 or La2 reaches to the lamp life end, a protective action is made as follows. That is, when the lamp reaches its lamp life end as a result of the depletion of the negative thermion radiating material (emitter) coated on the filaments, the lamp voltage of the discharge lamps La1 and La2 increases than in a normal condition. With this result, the voltage induced at the auxiliary winding N4 of the leakage transformer LT! increases so that the detection circuit 20 gives an abnormality detection signal to the control circuit CNT in response to the voltage induced at the auxiliary winding N4 exceeds a threshold. The control circuit CNT responds to the abnormality detection signal for activating the inverter circuit to intermittently oscillate, thereby effecting a protective action of reducing the stress on the circuit.
(Second Prior Art)
FIG. 2 shows a circuit diagram of another prior art which is identical in configuration to the circuit disclosed in FIG. 15 of a Japanese Patent Publication 2000-100587. The second prior art differs from the first prior art in that the inductor L2 forming the step-down chopper circuit is omitted, that diode 12 has its anode connected to a connection point of smoothing capacitor C10 and diode 13 and has its cathode connected to a connection point of the primary winding N1 of the leakage transformer LT1 and capacitor C3 in order to share the leakage transformer LT1 with the step-down chopper circuit, and that an output regulation circuit 21 is added in compensation for a large characteristic variation of a driving transformer T2. The output regulation circuit 21 includes a switching element Qb realized by a bipolar transistor connected across a control voltage source E through a variable resistor VR and a collector resistor Re. The switching element Qb has its base connected through a resistor Rd to a point between a resistor Rc and a capacitor Cb which are connected in series between the connection point of the switching elements Q1, Q2 and the negative pole of the control voltage source E. Connected between the output terminal of the control circuit CNT and the negative pole of the control voltage source E is a series combination of a diode Da, a resistor Ra, and a switching element Qa of bipolar transistor. The switching element Qa has its base connected through a base resistor Rb to a connection point of collector resistor Re and variable resistor VR. Further, a capacitor Ca and a diode Db are connected in parallel across the series combination of the switching element Qb and the collector resistor Re, while a diode Dc is connected in a base-emitter path of the switching element Qb. While the one switching element Q2 is off, capacitor Cb is charged through resistor Rc so that switching element Qb is caused to turn on in response to the voltage increase across capacitor Cb, thereby turning off the switching element Qa and giving no influence on the operation of the inverter circuit. When the switching element Q2 turns on, the switching element Qb is turned off so that the control voltage source E acts to charge capacitor Ca through variable resistor VR. As the voltage across capacitor Ca increases, the switching element Qa responds to turn on, thereby causing the switching element Q2 to turn off. Accordingly, it is made possible to regulate the on-period of switching element Q2 by varying the resistance of the variable resistor VR to thereby maintain the output substantially at a constant level irrespective of the varying characteristic of the driving transformer T2. Also this prior art has the same protective action as is made in the first prior art when the lamp life end is reached.
In the second prior art, however, the inclusion of the output regulation circuit 21 brings about an asymmetry (unbalance) of the on-period of the switching elements Q1 and Q2 in the normal lamp operating condition, whereby a DC voltage will be applied to capacitor C9 connected in series with the discharge lamps La1 and La2. With this result, the DC voltage of the charged capacitor C9 will be superimposed upon the high frequency output of the inverter circuit in the normal lamp operating, leading to a problem of causing a cataphoresis phenomenon particularly at a low temperature.
In order to solve the problem, it might be reasonable to remove capacitor C9 connected to the secondary of the leakage transformer LT1. However, this would causes another problem. That is, as the discharge lamp reaches the lamp life end, capacitor C9 accumulates an increased voltage so that the lamp voltage of the lamp of negative resistivity increases to make a great difference in the lamp voltage between the normal operating condition and the lamp life end condition. Such lamp voltage difference is utilized for detection of the lamp life end. However, in the absence of capacitor C9, the lamp voltage would make only a small difference between the normal operating condition and the lamp life end condition, making it difficult to detect the lamp life end particularly at a high temperature environment.
The present invention has been achieved in view of the above problem and has an object of providing a discharge lamp driving device which is capable of detecting the lamp life end reliably at either low or high temperature environment for circuit protection, yet preventing the cataphoresis phenomenon.
The discharge lamp driving device in accordance with the present invention includes a rectifier which rectifies an AC source voltage, a smoothing capacitor which smoothes out a pulsating output of the rectifier, an inverter circuit having one or more switching elements for conversion of the smoothed DC output made through the smoothing capacitor into a high frequency output, and a load circuit including a resonance circuit and a discharge lamp and being supplied with the high frequency output from the inverter circuit, an output transformer having a primary connected to an output end of the inverter circuit and having a secondary connected to one filament end of the discharge lamp, an impedance element inserted between the other filament end of the discharge lamp and a node having no high frequency amplitude, and an abnormality detection and protection means which detects an amplitude of the high frequency output flowing through the discharge lamp and the impedance element in order to make the circuit protection when the detected amplitude exceeds a predetermined threshold.
The abnormality detection and protection means judges the lamp life end of the discharge lamp when the amplitude of the high frequency output flowing through the discharge lamp and the impedance element exceeds the threshold. Since the impedance element is inserted between the other filament end of the discharge lamp and the node having no high frequency amplitude, reliable detection of the lamp life end can be made for the circuit protection at either low or high temperature environment. Further, since there is no need to connect a capacitor on the secondary of the output transformer, the cataphoresis phenomenon can be prevented.
In a preferred embodiment, the impedance element is inserted between the other filament end of the discharge lamp and a positive input terminal of the inverter circuit.
The impedance element may be inserted between the other filament end of the discharge lamp and a grounded input terminal or output terminal of the inverter circuit.
A plurality of the discharge lamps can be connected in series on the secondary side of the output transformer.
Each impedance element inserted between the filament of each of the individual discharge lamp and the node having no high frequency amplitude is preferred to have substantially the same impedance value.
In case where the plural discharge lamps are connected in series on the secondary side of the output transformer, the impedance element is inserted between the other filament end of at least one discharge lamp and the positive input terminal of the inverter circuit, while another impedance element is inserted between the other filament end of at least another discharge lamp and the grounded input terminal or output terminal of the inverter circuit.
In case where the plural discharge lamps are connected in series on the secondary side of the output transformer, the abnormality detection and protection means is set to make the circuit protective action when the amplitude of the high frequency output flowing through anyone of the discharge lamps and the impedance element exceeds a predetermined threshold.
Also in case where the plural discharge lamps are connected in series on the secondary side of the output transformer, the abnormality detection and protection means may be configured to detect the amplitude of a potential at a connection point of the filaments of the plural discharge lamps and also detect the amplitude of the high frequency output flowing through at least one discharge lamp and the impedance element such that it can make the circuit protective action when either or both of the amplitudes exceeds a predetermined threshold.
Further, in case where the plural discharge lamps are connected in series on the secondary side of the output transformer, the abnormality detection and protection means is configured to detect the amplitude of a potential at a connection point of the filaments of the plural discharge lamps such that it makes the circuit protective action when either of thus detected amplitude or the amplitude of the high frequency output flowing through at least one of the high-voltage and low-voltage side discharge lamps and the impedance element exceeds a predetermined threshold.
The impedance element may include a resistor, capacitor, and a series combination of a resistor and a capacitor.
When the inverter circuit is of a self-excited type, at least a portion of a driving circuit for driving the inverter circuit can be shared with components of the abnormality detection and protection means, enabling to reduce the number of the circuit components.
Still further, the impedance element can be shared with the resonance circuit included in the load circuit for reducing the number of the circuit components.